There are a variety of games, toys, and interactive learning devices in which a stylus is used to point to a region on a surface in order to input data or questions. There are several technologies to determine the position of a stylus on a sensing surface. One approach is to embed an array of pressure sensitive switches in the sensing surface, such as membrane switches. However, conventional membrane switches have limited resolution. Another approach consists of arrays of capacitive or inductive elements whose impedance is altered by bringing the stylus into contact with the surface. However, a disadvantage of this approach is that a large number of pixel elements are required to achieve a high resolution. Moreover, since capacitive and inductive effects are typically small, the stylus must be brought into close proximity to the pixel in order to obtain a strong position signal.
In many applications it is desirable to be able to determine the position of a stylus disposed a short distance away (e.g., 1 mm to 2 cm) from an electrically active surface. In many consumer products it is desirable to protect electrically active elements with a protective layer of plastic which is thick enough to provide both mechanical and electrical insulation. The insulating material, such as a layer of plastic, may also be patterned with numbers, indicia, symbols, and drawings which facilitate the user inputting data by pointing to a number, indicia, symbol, or drawing disposed on the surface of the plastic. Other applications include systems in which the number, indicia, symbol, or drawing is disposed on a top (open) page of a booklet. The position of a pointer disposed on the open page of the booklet may be sensed even though the pointer is separated from the active surface by the thickness of the booklet.
An electrographic sensor unit and method based upon a geometric algorithm that is described in U.S. Pat. No. 5,686,705 “Surface Position Location System and Method” and U.S. Pat. No. 5,877,458 “Surface Position Location System And Method,” which is assigned to the assignee of the present invention. According to the teachings of U.S. Pat. Nos. 5,877,458 and 5,686,705 the position of a stylus is determined by calculating the intersection point of equipotential lines based upon the measured signal strength received by the stylus. The contents of U.S. Pat. Nos. 5,686,705 and 5,877,458 are hereby incorporated by reference in the present application.
FIGS. 1-4 show the general principals of the geometric location method of U.S. Pat. Nos. 5,686,705 and 5,877,458. FIG. 1 is a simplified geometry illustrating the basic principles of operation. As shown in FIG. 1, a two or three dimensional conductive surface has a selected resistivity. In the embodiment of FIG. 1, three electrical contacts 12, 14, and 16 are connected to conductors 24, 26, and 28, respectively, to a processor 30. Also connected to processor 30 is conductor 18 with stylus 20 having a tip 22 for the user to indicate a position on the surface 10 that is of interest to the user. As shown in FIG. 2, when a user selects a point, P, on resistive surface 10, a series of field potential measurements are performed to calculate the position of the stylus. A DC offset value is determined with no radio-frequency (rf) signals applied to any of the contacts 12, 14, and 16. A second measurement is made by applying an equal amplitude rf signal to all three contacts 12, 14, and 16, and processor 30 measures the full-scale signal value via stylus 20. A third measurement is made by applying an rf signal to one of the contacts, such as contact 12, with a second contact grounded, such as contact 14. The signal measurement made by stylus 20 will lie somewhere along an equipotential line between those two contacts (i.e., line X in FIG. 2). A fourth measurement is made by applying the signal to, and grounding a different pair of contacts, say 12 and 16, and the signal measurement made with stylus 20 which will be somewhere along an equipotential line between those two contacts (i.e., line Y in FIG. 2) with the position of the stylus 20 being the intersection of lines X and Y. For the purposes of illustration, lines X and Y are shown as straight lines. More generally the actual position of the stylus on the surface can be determined using mathematically or empirically determined models of the signal level gradients for the surface material with curved equipotential lines.
FIG. 3 illustrates an embodiment of an electrographic sensor system of U.S. Pat. No. 5,877,458 having a rectangular shaped piece of conductive material as sheet 100. Affixed near the edge of sheet 100, and making electrical contact thereto, are contacts 102, 104, and 106. Connected between contacts 102, 104, and 106 on sheet 100 and contacts 126, 128, and 130 of signal generator 122, respectively, are electrically conductive leads 108, 110, and 112. Signal generator 122 includes an rf generator 124, amplifier 134, and switches 132 and 136 to determine which signals are fed to contacts 126, 128, and 130. The position of switches 132 and 136 is controlled via cables 138 and 140, respectively, from microprocessor 142 to select which contacts 102, 104, and 106 receive an normal or inverted rf signal.
Stylus 116 contains a receiving antenna and is coupled to signal measurement stage 120 via cable 118. The signal is demodulated and turned into a digital signal via demodulator 144 and analog to digital converter (ADC) 146. ADC 146 presents the digitized signal to microprocessor 142. Microprocessor 142 includes RAM 145, ROM 147, a clock 148 to contain information related to the position that has been pre-stored along with an audio card 150 and speaker 154 or monitor 152 to output information on the selected area.
When an rf signal is coupled to one or more of the contacts 102, 104, and 106 the signal radiates through the conductive material of sheet 100. Between a given set of energized contacts, such as contacts 102 and 104, a signal level equipotential map 114A exists because of the distributed resistance in the conductive material of sheet 100. The signal level equipotential map includes the shape and values of the equipotential lines and may be stored in the memory of the microprocessor or the ROM 147. The shape of the these equipotential lines may, in principal, be calculated by finding the unique solution of mathematical equations or may be determined empirically. Additionally, there will be a signal equipotential map for other sets of energized contacts, such as equipotential map 114B for energized contacts 102 and 106. The measurement of the signal strength received at the stylus for a particular set of energized contacts may be used to calculate which equipotential line the stylus lies on. The measurement of two sets of energized contacts with substantially orthogonal equipotential lines permits the position of the stylus to be calculated, as indicated by point P of FIG. 3.
FIG. 4 has similar elements as for FIG. 3 as applied to a globe having two hemispherical conducting surfaces 701 and 702. Insulating map surfaces 601 and 602, containing details of world geography, are shaped to house hemispherical surfaces 701 and 702. Hemisphere 701 has contacts 710, 711, and 712. Hemisphere 702 has contacts 740, 741, and 742. Switches 770, 771, 772, and 773 along with cables 730, 750 and leads 760, 761 of signal generator 722 are configured so that each hemisphere 701 and 701 is driven in a manner similar to that of sheet 100. However, the equipotential maps for a hemispherical surface energized by two contacts, such as contacts 710 and 711, is typically more complex than for sheet 100 because of the spherical geometry. Additionally the mathematical algorithms must be calculated in spherical coordinates.
The electrographic apparatus and method of U.S. Pat. Nos. 5,686,705 and 5,877,458 has many applications, such as interactive globes. One advantage of the electrographic sensor technology of U.S. Pat. Nos. 5,686,705 and 5,877,458 is that the mechanical construction is comparatively simple and inexpensive. The conductive surface 100 or 701, 702 may be formed using a variety of deposited or coated materials. The position resolution is superior to many competing technologies, making it desirable for a variety of educational toys. For many applications the position of the stylus may be calculated to within several millimeters, making the electrographic apparatus of U.S. Pat. Nos. 5,686,705 and 5,877,458 useful in a variety of interactive games, such as the EXPLORER GLOBE™, sold by LeapFrog Toys of Emeryville, Calif. However, the inventors of the present application have recognized several drawbacks to the electrographic apparatus of U.S. Pat. Nos. 5,686,705 and 5,877,458. One drawback is that significant electronic memory and computing time is required to perform the mathematical calculations. In order to convert measured signal strengths into position data an equipotential map or equation is useful. The equipotential lines between energized point contacts on solid two-dimensional surfaces, or surfaces having uniform resistivity, have non-linear, non-parallel and curved contours which lead to complicated algorithms for position determination. The complicated algorithms, in turn, result in relatively expensive and slow electronics. Additionally, in some topologies, such as that of hemisphere 701, the curved geometry further complicates the calculation of the shape of the equipotential lines. Consequently, significant memory and computing time is required to perform each position calculation.
Another drawback with the electrographic location position sensing system of U.S. Pat. Nos. 5,686,705 and 5,877,458 is that the position sensing resolution tends to degrade towards the edges and corners of the active surface. The position sensing method of U.S. Pat. Nos. 5,686,705 and 5,877,458 is based upon calculating the intersection of equipotential lines from different pairs of energized contacts. However, the equipotential lines tend to be parallel near the edges and corner of common surface shapes. As is well known, it is difficult to obtain accurate measurements of the position of a point based upon the intersection of two nearly parallel lines because a small empirical variation in measured data produces large variations in the calculated intersection point. Consequently, position resolution will tend be poor in regions of surface 100 or 701 where the equipotential lines of different pairs of energized contacts are nearly parallel. Experiments by the inventors with hemispheres 701, 702 similar to those shown in FIG. 4 indicate that there is a region around the rim of a hemisphere 701, 702 with greatly reduced position resolution capability, which the inventors attribute to nearly parallel equipotential lines near the edge of a hemisphere 701, 702. This makes it difficult, for example, to design an interactive learning globe in which the user can point to small countries located close to the equator (e.g., Equatorial Guinea) to obtain information on the country. Similarly a rectangular conductive surface, such as surface 100, there also tends to be a region of reduced resolution near the edges of surface 100, making it difficult, for example, to identify small countries or regions located on the edge of a planar map. Further the equipotential lines for a planar surface of uniform resistance are curved and generally less orthogonal and therefore more complex than is desirable as illustrated in FIGS. 5A and 5B. This results in complex and slow mathematical algorithms.
Common techniques to form a continuous resistive coating on a surface 100 or 701 result in significant spatial variations in thickness and/or resistivity. In a single fabrication lot there can be substantial variations in the electrical resistance of each surface. This variation in resistivity across the sensing surface can significantly effect the contours of the equipotential lines. Therefore, it is necessary to compensate for those effects with a two-dimensional algorithm that leads to complex and time-consuming manufacturing processes. Consequently, a large number of data points are required to accurately map the equipotential lines. Additionally, a large amount of data must typically be stored in an equipotential map. This increases product cost.
An electrographic position sensing system using a similar calculation to U.S. Pat. Nos. 5,686,705 and 5,877,458 is desirable because of the potential for high accuracy, low manufacturing cost, and comparatively simple construction. However, previously known electrographic position sensing systems suffer from the problems of reduced resolution along edge regions because of the substantially parallel equipotential lines disposed along edge regions, the requirement of significant computational memory and computing time to calculate a position based upon complicated equipotential contours, and the need to perform complicated calibration procedures to map the equipotential lines.
What is desired is an improved electrographic apparatus and method providing improved control of the equipotential signal contours.